In order to minimize the amount of interference between uplink signals transmitted by different mobile devices to a network component such as an access node, it is often desirable to ensure that the uplink signals originating from the different mobile devices are time-synchronized when they arrive at the network component. A network component may control the mobile devices' timing so that all uplink signals arrive at the network component in an essentially synchronized fashion. Different mobile devices may be at various locations throughout the cell and have varying propagation times for their uplink transmissions. In order to ensure uplink signal synchronization at the network component, each mobile device applies an individual timing advance to its uplink transmissions. Typically, the timing advance is applied relative to the downlink reception timing that is measured by the mobile device.
A mobile device may receive regular timing advance adjustment instructions from a network component in the form of timing advance command (TAC) medium access control (MAC) control elements. These instruct the mobile device to adjust its timing advance in either a positive or negative direction. In order to determine the appropriate timing advance adjustments to issue to each mobile device, the network component measures transmitted uplink signals from each mobile device and then determines how the uplink timing has changed relative to the uplink timing that is desired at the network component. When the uplink timing of a particular mobile device's transmissions (as measured at the network component) has changed by a certain amount, the network component will issue a TAC to the mobile device in order to “fine tune” the mobile device's uplink synchronization.
Conventionally, this measurement process at the network component may require some form of transmission from the mobile device so that timing measurements can be performed. For example, if a mobile device is performing regular data (e.g. on the E-UTRA (Evolved Universal Terrestrial Radio Access) Physical Uplink Shared Channel) or control feedback (e.g. on the E-UTRA Physical Uplink Control Channel) transmissions on the uplink, then the network component can use those transmissions to conduct the necessary timing synchronization measurements. Conversely, a mobile device that is relatively idle in a traffic sense (i.e. has no data to transmit), but which is still connected to the network and therefore considered to be active, may not be performing such uplink transmissions. It may therefore be necessary for such a mobile device to make periodic SRS (Sounding Reference Signals) transmissions on the uplink. These are channel sounding transmissions that allow the network component to measure the mobile device's uplink transmission timing when other uplink transmissions are not available or are not performed. Such additional signaling can use power and reduce battery life, as well as consuming additional cell resources that could otherwise be used for data transmission.